I get a persistent high pitched noise in all my Canon SX260 videos. It seems to be a hardware issue. From what i read the culprit is Auto gain control which boosts internal electronic noise. I've had this also in my older Powershot A610 but it is really bothering me now.
Although I can eliminate the noise during post-processing using sound equalization in Sony Vegas and turning gain to minus infinity for the specific two noise frequencies which bother (around 7,800 hz and 15,500 hz) it means I have to re-encode every clip - a thing I want to avoid.
Can you find a way to cut certain frequencies when recording using CHDK?